In history class, the girl to boy ratio is $6$ to $2$. If there are a total of $40$ students, how many girls are there?
A ratio of $6$ girls to $2$ boys means that a set of $8$ students will have $6$ girls and $2$ boys. A class of $40$ students has $5$ sets of $8$ students. Because we know that there are $6$ girls in each set of $8$ students, the class must have $5$ groups of $6$ girls each. There is a total of $30$ girls in history class.